1. Field of the Invention
The present invention relates to a zoom lens suitable for use in an image pickup apparatus such as a digital still camera, a video camera, a film camera, a TV camera, or a surveillance camera.
2. Description of the Related Art
In recent years, image pickup apparatuses (cameras), such as a video camera and a digital still camera, using a solid-state image pickup element have been sophisticated and reduced in size. As an optical system for use in the image pickup apparatuses, there is a demand for a compact zoom lens having a wide viewing angle, and a high zoom ratio. A negative lead type zoom lens is known as a zoom lens that meets this demand. In a negative lead type zoom lens, a lens unit having a negative refractive power is provided—with respect to the image pickup element (image side)—closest to an object to be photographed (object side).
As a negative lead type zoom lens, U.S. Pat. Nos. 7,215,482, 7,450,318, and 7,295,381 disclose compact three-unit zoom lenses each including, in order from an object side to an image side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, and a third lens unit having a positive refractive power. In the zoom lenses of the above-referenced patents, the first lens unit having a negative refractive power consists of two lenses, that is, a negative lens and a positive lens.
To reduce the total size of the zoom lens and to increase the zoom ratio, it is generally effective to reduce the number of lenses in the lens units of the zoom lens and to increase the refractive powers of the lens units. Unfortunately, if the refractive powers of the lens units are simply increased for a higher zoom ratio, aberration variation increases during zooming, and it is therefore difficult to achieve a high optical performance over the entire zoom range. Although the viewing angle can be increased while properly correcting aberrations by increasing the number of lenses in the lens units, this increases the total size of the zoom lens.
For this reason, to achieve a compact size, a wide viewing angle, and a high zoom ratio in the negative lead type three-unit zoom lens, it is important to appropriately set the lens configurations and refractive powers of the lens units. In particular, the lens configuration and refractive power of the first lens unit having a negative refractive power are important.